Democrats in Congress believe retired Lt. Gen. Michael Flynn illegally concealed more than a dozen foreign contacts and overseas trips during the process of renewing his security clearances, omissions they considered so serious they forwarded their findings to special counselRobert Mueller.

"It appears that General Flynn violated federal law by omitting this trip and these foreign contacts from his security clearance renewal application in 2016 and concealing them from security clearance investigators who interviewed him as part of the background check process," Reps. Elijah Cummings and Eliot L. Engel, both Democrats, wrote in a letter to Flynn's attorney obtained by ABC News on Tuesday.

Flynn is a decorated military officer who served as director of the Defense Intelligence Agency from 2012 until his retirement in 2014. He was only out of the spotlight briefly. He joined the Trump campaign as an adviser in 2016, and President Trump would later name Flynn as his first national security adviser. He was forced to resign, however, after just 24 days on the job, when it was revealed that he misled Vice President Mike Pence about his conversation with Russian officials during the presidential transition.

Cummings told ABC News that Flynn's foreign contacts - which involved high ranking foreign officials and business executives - were so numerous they could not have been inadvertent omissions or incidental contacts.

"He has, over and over again, omitted information that he should have disclosed," Cummings said. "It's not an aberration, and that's clear.

Reached yesterday, Flynn's lawyer declined to comment. ABC News tracked down Flynn this summer at a beach in Newport, Rhode Island -- his home town. "I'm just having a great time with the family here," Flynn said. "I'm doing good, [but] I'm not going to make any comments."

The omissions are a serious matter - and not just for Flynn. While Cummings said intentionally omitting foreign contacts when applying for security clearance can carry a five-year prison term, he also acknowledged that penalties are rarely so severe. The leverage alleged transgressions provide, however, could prove useful to prosecutors seeking to use the threat of prosecution to compel Flynn's assistance in the broader investigation into Russian interference in the 2016 campaign.

Former FBI Director James Comey provided a window into that strategy during his three hours of testimony before the Senate Intelligence Committee earlier this year.

"There is always a possibility if you have a criminal case against someone and you bring them in, squeeze them, flip them, [that] they give you information about something else," Comey said.

Cummings told ABC News he believes the omissions raise questions about whether Flynn intended to hide his consulting work so he could continue to push for the nuclear venture with Russia from inside the White House.

"We want to know if this is an ongoing proposition that they are still involved in," Cummings said. "We just want to see exactly what part Lt. Gen. Flynn played, and may very well continue to play."

Investigators with the House Government Oversight Committee, which has jurisdiction over questions about the security clearance process, have used the retired general's past public remarks against him.

In an August 2016 interview with NPR, Flynn said, "I have spoken to friends in Saudi, friends in Egypt, friends in Kuwait, friends in Qatar, friend in UAE." At a speaking engagement that same month, Flynn remarked about his frequent conversations with Egyptian president Abdel Fattah el-Sisi, saying "President al-Sisi is a good friend of mine.

Cummings, whose staff has reviewed Flynn's security clearance forms and reports from those who interviewed him as part of the clearance review process, said the retired general reported some contacts, but none that match those public remarks.

These latest alleged omissions are just the latest to make trouble for the retired general. Flynn failed to declare a December 2015 trip to Russia, where he sat next to Russian President Putin, and was paid $33,000 for his appearance. In March 2017, Flynn submitted a late filing with the Department of Justice under the Foreign Agent Registration Act (FARA), revealing that his consulting firm, Flynn Intel Group, was paid $530,000 for three months of work on behalf of a Dutch firm owned by a Turkish businessman with close ties to the Turkish government.

Flynn's work for Turkey also remains the subject of additional scrutiny. Of interest to federal agents, according to people interviewed by the FBI, is his alleged role in a bizarre, unrealized proposal first reported by the Wall Street Journal to kidnap Turkish dissident cleric Fethullah Gulen, who is living in exile in rural Pennsylvania and suspected of involvement in a failed coup attempt.

Gulen, who has denied involvement in the coup attempt, has lived legally in the Pocono Mountains since 1999, and the Turkish government has been financing efforts to persuade the U.S. government to return Gulen to Turkey for years.

Former CIA director James Woolsey confirmed to ABC News he was at a meeting in which Flynn allegedly raised the idea.

"It became clear to me that, they were seriously considering a kidnapping operation for Gulen, and I told them then that it was a bad idea, it was illegal," Woolsey said. "I won't say that they had firmly decided to do that. But they were seriously considering it."

Flynn's lawyer has said there was no such discussion, calling them categorically "false." In mid-July at a press conference, the Turkish ambassador to the U.S. also denied the notion of a kidnapping plot.

"There's no truth to that," he said, adding that the government was following "traditional" procedures to have Gulen extradited "through the legal channels."

Woolsey, however, told ABC News that he was so alarmed by the consideration of such a plan that he said he reported it to a senior government official he hoped would be able to stop it.

Sen. Ben Cardin, the ranking Democrat on the Senate Foreign Relations Committee, told ABC News he found the allegations there was discussion of attempting to remove Gulen from the U.S. without a formal extradition process "outrageous."

"I think there's a lot of information about Mr. Flynn, General Flynn, that we don't know," Cardin said. "And I think it's going to be very interesting as the investigation continues, as to whether we learn more information about contacts that he made, or other individuals that may be involved with him. ...And that's why the Mueller investigation is particularly important."